The object of this collaborative study is to determine whether a dietary supplement of n-3 polyunsaturated fatty acids (PUFA) will decrease the restenosis rate percutaneous transluminal coronary angioplasty (PTCA). This procedure has proven very useful in relieving the symptoms and improving myocardial perfusion in coronary artery disease; however, its "Achilles heel" is the 25 to 35 percent restenosis rate that occurs within 6 to 8 months post PTCA. Reduction in restenosis rates will be tested in a double-blind, randomized, placebo-controlled, prospective study in patients undergoing angioplasty of native coronary arteries. All subjects will receive usual clinical therapy including 324 mg of aspirin per day and will be instructed in the American Heart Association phase I diet; half will receive n-3 PUFA (the intervention group) and the other half will receive a similar amount of vegetable oil (the placebo group). Adherence to diet and capsules will be maintained and monitored by monthly interview and analyses of plasma and red blood cell phospholipid content of n-3 fatty acids as well as capsule count. If a beneficial effect on the restenosis rate is achieved - and we are seeking a 33% reduction in the n-3 intervention group - it will have a very important impact on the management of the many patients undergoing PTCA. Since after the first few weeks, during which thrombi cause the failures, the restenosis process is pathologically an accelerated atherosclerosis, we expect the results to have broader implications for the prevention of atherosclerosis and coronary artery disease, the major cause of death in the U.S. and in Western industrialized countries. There is much confusion in the minds of both the public and physicians regarding possible beneficial health effects of fish oil. This study is undertaken to establish factual information on which to judge this issue.